1. Field of the Invention
This invention relates to variable capacitors and, more particularly, to an arrangement for an adjustable low-loss capacitor which can withstand high shock forces.
2. Description of the Prior Art
Adjustable or variable capacitors often find extensive use in electronic circuitry. Capacitors may be used to provide a tuning function or a trimming function. The latter is often required when unavoidable variations in the values of other components in an electronic system call for a capacitance value slightly different from that originally contemplated in order for the system to perform as designed.
One type of variable capacitor is that shown in U.S. Pat. No. 3,469,160 (issued Sept. 23, 1969 to J. E. Johanson) and reissued (on Sept. 16, 1980) as U.S. Pat. No. Re. 30,406. The capacitor described therein has a stator unit and a rotor unit, each comprised of an electrode having a series of coaxially arranged cylindrical conductive tubes. The tubes on the stator unit are rigidly connected by a hollow, insulating member to one end of a rotor bushing which has a series of internal threads. The rotor unit is comprised of an externally threaded screw with the rotor tubes mounted to one end, i.e. the electrode end, of the screw. The other end of the rotor unit, i.e. the adjustment end, has a slot into which an operator, for example, can insert an end of a screwdriver blade in order to turn the rotor unit and thereby adjust the capacitance. As the rotor unit is precisely threaded into the rotor bushing, the rotor tubes enter and become increasingly interleaved with the stator tubes. The resulting capacitance obtained with this arrangement is a linear function of the longitudinal, i.e., axial, displacement of the rotor unit and can be set with a high degree of precision to any value falling within the adjustment range. With this arrangement, maximum capacitances of between 5 to 500 pico-farads are commonly obtainable with an adjustment range of approximately 90% (or more) of the maximum capacitance.
To firmly hold the rotor unit in a desired position, added friction is imparted between each of the mating threads of the rotor unit and rotor bushing. This is readily accomplished by cutting two opposing slots into the rotor unit, and then pre-compressing these slots prior to threading the rotor unit into the rotor bushing, as shown and described in the previously noted U.S. Pat. No. Re. 30,406. A variant of this arrangement is shown in U.S. Pat. No. 4,305,113 (issued Dec. 8, 1981 to A. B. Shai) and involves the use of three slots whose inner edges are arranged to form an equilateral triangle about the center axis of the rotor unit.
Although the friction imparted to the mating threaded elements which secure the relative position of the rotor unit with respect to the stator unit in either of these devices is sufficient for most operating environments, it may be inadequate in an environment where the capacitor is subjected to high shock forces, for example, in excess of 25,000 g's. As a result, the actual capacitance is likely to deviate from the desired preselected value whenever a shock force of this magnitude is encountered. In fact, successive applications of a force of this magnitude could cause the actual value of the capacitance to depart significantly from the desired preselected value, thereby rendering the use of this capacitor unsuitable for such applications as ballistics, aircraft and space applications where shock forces of this magnitude are expected to occur.